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Why does it feel like a plane accelerates just before touching down?
On commercial flights it often feels like the plane gets faster just before touching down. What creates that sensation?

Are you sure it's not just subjective?  It's been a while since I've flown commercial, but I don't remember any such feeling.  And I don't notice a "feeling" in light airplanes, other than the visual sensation from being closer to the ground.

@jamesqf I am not sure what a feeling could be, if not subjective.

The aircraft flares just before touching down. It descends with a constant velocity, and just before touching down pulls the nose up to reduce the descent. This results in a higher angle of attack, more lift, and a vertical deceleration of the airplane. A passenger perceives this vertical deceleration as a force. Direction of the force is straight down and the aircraft is nose up, you're leaning back, so there is a component of (gravity + vertical deceleration) that pushes you into the back of your seat.
This same effect is used in flight simulators with a motion base. Upon accelerating, the simulator pushes forward like the aircraft does, but also slowly rotates backwards so that the pilot feels sustained seat back pressure. He cannot see the leaning back angle inside the simulator because the horizon of the projected outside visual image does not move. As happens in the aircraft: look forward in the aircraft and your view of the inside of the aircraft is always the same, irrespective of aircraft angle.

So for your eyes there is no apparent tilt, the inner ear senses rearwards force, and there is increased pressure from the seat backrest. The brain translates this into perceived forward acceleration.

In other words: you are being accelerated downwards into your seat by gravity as well as the seat being accelerated upwards into you by the flare, but you think that your seat is being accelerated forwards into you.

Gravity - yes. Additional upward acceleration from flare - yes. But your seat is tilted backwards, that is why the back feels some of the vertical acceleration.

While highly upvoted, this answer may not be correct. When the flare occurs, the nose rotates up which should decrease the pressure on the passenger back due to inertia. This should create a feeling of being decelerated in the forward direction. An illustration would be helpful.

@mins During rotation the flare action would contribute a small amount of reduction in the apparent "backwards" force due to the rotating gravity vector, but the movement is slow enough that the effect is minimal.  The dominant force would be gravity.  6 degrees of flare produces about 1m/s^2 apparent acceleration force into the back of the seat - about the same as the acceleration you would feel in a normal car accelerating from a stop (driving conservatively).

It depends on the centre of rotation. If this is located above the passenger, the rotational acceleration actually helps the back pressure (this is the way it's done in the simulator, with centre of rotation at the pilot's head, and rotation taking place slowly so the inner ear does not detect it). It depends on whether you're seated forward or aft of the centre of rotation: if you're sitting a couple of meters in front of it, the rotational acceleration actually helps. But after the rotation is completed, the resulting angle creates the back pressure feeling.

@mins Rotational acceleration is pretty small anyway. I'm assuming you're referring to rotational acceleration, not velocity, since you mentioned inertia.

@mins Actually, centre of rotation might very well be above the passenger seat since we're adding to the aircraft velocity vector. Passengers fwd of CoG go up due to rotation, but down due to landing speed * sin (glide slope).  Passengers aft of CoG go down with ω * distance + landing speed * sin glide slope. But you're right, a picture is worth 1000 words, I'll source a picture of a seated person in a back leaning chair with the gravitation and acceleration vectors.

@Koyovis: I don't think a leaning chair will help, it's not about the existence of a gravity component backwards in a static situation (nobody will challenge that), it's about how all forces combine during the flare. As for the rotation under elevator deflection being around a center above the passenger seats in a A320... I'm lost :-)

@mins at 0:50 in this (https://www.youtube.com/watch?v=sPyXLmg87s0) video: see the aircraft flare without the cockpit going up. I would call the cockpit the centre of rotation.

@JörgWMittag Gravity is exerting a downward force on you, but you're not actually being accelerated downward. On the contrary, the net force exerted on you is upwards, so you're acceleration is upwards (i.e. your rate of descent is decreasing in magnitude.)

Aside from the sensory effects already mentioned, there are also occasions when the pilot does indeed add a bit of power shortly before touchdown.  See, for example, comments #5, #16, and #17 from Adding Power Just Before Touchdown.

Yes, the sound of the engines spooling up will further add to the sensory inputs.

Visual perception of your perspective; as the aircraft gets closer to the ground your field of view constricts and you are closer to terrain and other structures, making them appear to move faster past the jet than at altitude.

This is not only a visual sensation, because I recall this same feeling apart from looking out the window. It feels as if you are being pushed back in your seat.

As the plane descends into ground effect, it may actually accelerate if the engines are producing enough thrust, since in ground effect the plane requires much less power to keep "flying".  Power from the engines will translate into speed, if not height.
I don't know if it happens often in real world landing configurations, but it's certainly possible.  To make it happen for sure, set the power so that the plane is just barely descending, and when it gets into ground effect, it will certainly speed up measurably.

I think what you're feeling is the brief float due to the ground effect. Within about 10 feet of the ground the air being forced down by the wing encounters the resistance of the ground. It actually arrests the downward motion unless the angle is pretty steep and that float gives a slight sinking feeling that feels like acceleration. Sometime too the pilot will add a little throttle to transition out of the ground effect a little less abruptly.

Yes ground effect helps in arresting the downward velocity. But how does that transform into a sensation of forward acceleration?

You'd have to instrument it to see the small changes and there may be a small decrease in drag. What we feel in the air isn't what necessarily what is really happening which is why we fly by instruments and not feel in low visibility situations. Our senses fool us very readily.

when the flaps are lowered more power is applied to make up for the losses sometimes I fell a drop in speed and then a little surge in acceleration.

Welcome! Do you suggest the flaps are lowered just before touchdown?

It does accelerate just before touching down in case something goes wrong with the landing gear and the plane has to immediately take off again.

DV - I'm sorry for any confusion, but this is not what I am referring to. I am asking about a phenomena felt during a routine and successful landing.

Welcome to aviation.SE! Do you have any reference for this? It does happen during aircraft carrier landings, but that's a very special case and not really what this question is about.


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What is this red dashed line between door and cockpit window?
I noticed this dashed line located between the door and cockpit window on this 737-300 while boarding this exact aircraft at Ibiza airport. It seems to have text beside it but I did not have time to read it while boarding and it is too hard to read from the photo. Older photos of the aircraft with the same paint job do not have this suggesting that it was added recently. Wondering if it has anything to do with aligning air stairs or jet bridges. Does anyone know any other aircraft with a similar marking?

(Photo by Roberto Bianchi, source)

Can you find an image with a closer view of what you are describing?

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How airlines select new Captains...?
How does an airline select Captains for a new but different airplanes (ie, Boeing vs Airbus) that they just added to their fleet?
As an example, Delta has purchased many new Airbus A220. Since it is a new airplane and no one has any experience with it, do the pilots train the same for this new airplane and then based on their seniority one becomes Captain and the other a FO...or...they train for their respective position from the start?

Qualification for being a pilot-in-command in mutli-crew aircraft (i.e. captain) is independent of type rating (i.e. qualification to fly specific aircraft type). And similarly with the multi-crew rating required to fly a multi-crew aircraft at all (i.e. as F/O).
Therefore the airline selects some pilots and sends them to appropriate training to get the type rating. And those who were captains before can be captains on the new type too.
Since much of the experience applies to any aircraft, usually they'll select the more experienced captains from other types to become captains on the new type.
If the type is widely used elsewhere, the airline will also try to hire pilots from elsewhere who already have appropriate ratings, but obviously there is not much opportunity for that in a new type like A220 (formerly known as CSeries).

Just a minor point, it's not a guarantee that a captain on one type can be a captain on another right away - e.g an A320 captain might not have seniority over a 747 FO

@Ben, but he would still be qualified to be a captain, wouldn't he? Of course when the airline has more pilots qualified as captains than it needs for the scheduled flights, it will not roster the less experienced ones as captains, but the question is what happens when they don't have them.

I wanted to ask the very same question. So far this is the only answer, why is the downvote? Is it not true? Based on some common sense I would have guessed the same thing. It would be just impossible to operate new types (new type for an airline, not necesseraly a brand new type for everyone) because there would not be sufficient captain "supply" if Jan Hudec's answer was not true.

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Will VR make large professional simulators obsolete?
I believe that with the new breed of low latency VR displays such as the Oculus Rift, as well as associated control peripherals, the days of the large simulator are numbered. Crews can train together from different locations and each can be on their own moving chair, much cheaper to actuate than a full-hydraulics cockpit.
Will VR based solutions replace simulators with physical displays for FAA certification purposes?

This seems to be primarily opinion based. As someone who has an Oculus DK2 and P3D as well as DCS, in my personal experience, the technology is still miles away to be used anywhere near actual flight training. Still a lot of fun though.

@JustSid I disagree, the question itself does pose an opinion, but the answer itself will not be opinion based.  There are very real, easily explained reasons why this concept will not work. (as Ralph J has shown us)

Based on the variety of tasks that are performed in a simulator that VR would have great trouble accurately replicating, I think the answer is likely, no.  There IS a role for "simulators" that are more accurately referred to as "procedures trainers" in that they are non-motion, but this has nothing to do with VR, as those don't use all that much visual display anyway.  Networked wargaming is a great capability that VR can add to military simulations, but that's an addition to, not a replacement of, traditional simulators.
In a full-motion simulator, there are several things about the simulator that contribute to aircrew training, and the visuals are one important part.  The replication of the entire cockpit, including every switch, lever, display, keypad, and so on is another.  When I go to the sim and practice emergency procedures, each control reacts exactly as it does in the aircraft: where I look and where I reach to put my eyes and then my hand on each control, how hard I need to pull to get the desired effect, and so on.  And where I'm reaching with my sitting height and arm's reach is ever so slightly different than another pilot does.  We both get the same control, but it's farther below a tall pilot's eye line than a short pilot's. (Slightly over-simplified, but the basic point remains.)
We also do a fair amount of manipulation of the Flight Management Computer while in the simulator, which involves typing.  Not to say that typing isn't possible in VR, but it's not exactly the same as typing on a physical keyboard.  And we're using the same physical checklists and iPad EFB's that we do in the aircraft; not sure how well VR can mimic those.
So when you consider all the parts & pieces that make up the modern simulator & its considerable cost, the physical replication of the cockpit probably isn't something you're going to be able to remove in favor of VR.
For basic flying training, some of these sorts of factors are less important, as the basic flying skills you learn on one trainer transfer quite well to another trainer, even with a different cockpit layout.  But for basic flying training, not only the visual but also the vestibular and "seat of the pants" elements are more important.  While VR goggles like Occulus Rift are probably cheaper than several large display panels for providing the out-the-window visuals, these days high quality visual displays aren't the most expensive part of most simulators, and VR can't simulate the seat-of-the-pants sensation of accelerating, stopping, turbulence, and the like without some sort of motion actuators -- hydraulic or electric.
Now, for networked purposes (i.e. wargaming where Air Force elements from multiple bases across the country or globe can participate without the time & expense of getting to, say, Nellis AFB for a Red Flag), VR has amazing potential.  But to the extent that each pilot still has his own realistic cockpit & switches, what's driving the big gains is more the network than the VR... you can network an existing visual display with about the same hardware as you'd network the Rift devices.  To the extent that the "reality" of the precise cockpit can be sacrificed, then the VR probably makes for fabulous gains -- units practicing their tactics, rather than precise switchology. Sometimes that's a perfectly acceptable tradeoff; other times, not so much.
But until the tactile feedback of VR is good enough that I can use it to practice putting my hand on a lever, pulling it out of its detent in "this" direction with "this much" force required for "this much" travel, then pushing it down "this far" with "this much" force until it hits the stop "here," I really doubt that VR will replace the high-dollar flight simulators used in military and airline training.

A very important part of sim training is CRM and the human interactions cannot be replicated using VR (at least, not in the immediate future).

@Simon, most of the human interaction is talking and transferring voice is manageable. Good tactile feedback is the larger problem.

@JanHudec I agree that tactile feedback is the larger problem, but CRM cannot be used, practised and tested through voice alone.  In addition to needing to see what the other guy does, there are numerous subconscious cues that are not present in voice only.  It's the reason why using a phone when driving is so much more dangerous than talking to someone in the car.

A contributing factor to this landing with nose gear up was "the dissimilar pull force and pull length set on the flight simulator that was used for flight crew training of alternate landing gear extension procedures".  Tactile feedback matters.

Good answer and fully agree, with following additions: 1. Cost of the visual system is 15% of total cost of the simulator. 2. There are projects for including tactile feedback into the VR goggles: you press a button that feels right, but see the image in the goggles when you look at it. Still, VR lacks peripheral vision, amd imtroduces a weighty reminder that we're not in the air.

Great answer, but your second sentence is confusing…. I understand the difference between procedures trainers and full motion simulators, I’m just not sure what point the sentence is trying to convey.

@MichaelHall  In a procedures trainer, "tap on the switch" to activate it is (even if the real switch is a toggle or guarded or whatever) good enough, and flat-panel displays or VR with minimal haptic feedback can accomplish that. But that level of reality is just a fancy paper trainer and far short of what we get from actual flight simulators. The sentence is an introduction to the difference between that level of procedures trainer, and all the fidelity that's needed in a full simulator.

Ok, I get what you are saying then.  I might just explain briefly a little more about what a PT or VPT is, and how VR might be a better fit for that than for replacing full motion simulators.

Well, if you ask me in the current era of VR, my answer would be no.
But, It could be used as a pre-trainer for the real simulator to feel familiar.
Additionally, in the coming years, we could incorporate the tactile feedback using haptic suits which was the major concern raised by @Ralph J. You can even add how hard you have to pull a lever in that particular suit. In that era of VRs XRs, MRs hardware simulators could be alternative to the real simulator training for those people who do not have access to a trainer nearby or in a Covid-like situation ( the suits can be delivered to the concerned person on a lease basis).
VR can enable online training. Heck, we could make a Metaverse Aviation academy where you have to walk to the aircraft! But that's a distant story and Meta and other startups are working on it.

There is a difference between haptic feedback that can fake 'touched a thing' and the body position sense of proprioception for 'pulled lever back and to the stop'. A suit that can provide the force feedback strong enough to overcome your motion pulling/pushing something is going to also be strong enough to badly injure you when it fails.

@GremlinWranger, Nothing in an aircraft cockpit requires a level of actuation force that if applied to a human body would cause injury. It would be easy enough to use actuators with safe levels of resistance.

@MichaelHall  In the 737, when you push the throttles all the way to either stop, they STOP with a very definite "clank" and your hand knows with certainty that this is as far as they go. For a procedures trainer, dragging a throttle icon to the full aft position to practice "Throttle (affected engine) - Confirm - Idle" is probably fine, but it wouldn't feel even slightly like flying the actual airplane.

Or, another example: the fire handles. They unlock when illuminated, and can then be pulled up and then rotated. If the aren't unlocked (by an actual fire, fire test, or an awkward hard to reach unlock button under each one), you can pull until you break the panel, but the handle isn't coming up the inch (or less) it moves when unlocked. To realistically simulate that small but important detail, the "haptic feedback" would have to be able to resist a lot of force applied by the pilot without allowing any vertical movement at all. That would take a really strong feedback mechanism.

@RalphJ -  In your throttle example, how much force do trainees typically exert before realizing the levers are at the stop?  Enough to injure someone?!  Surely something much less than the point of injury or the yield strength of the quadrant would be enough to register the point that that is as far as they are intended to go...

@MichaelHall the issue is leverage, if you want to have that throttle 'stop' at arms length with a proper 'in detent' feel you need your arm mech to be pretty solid, which means heavy with power assist. This is going to be 'power loader from alien' not 'sleek body suit'. As a more extreme example imagine the mechanics of a suit that allows you to adjust your virtual seat height/position. At some point you always have to decide 'is this easier to do with the real thing' (or just accept the realism hit).

You brief the trainee that when the VR throttles lever are visually full forward and they feel a light “click” in the suit, that is the full forward detent.  Yes, it’s VR, you can keep moving your hand forward but you won’t go faster.  If they can’t grasp that simple idea they shouldn’t be a pilot.  You don’t need an Alien loader suit to resist them.

In one circumstance you may close the throttles slowly until they reach the stop with a tiny clink, but if you pull the back hard, the real thing will give you a hard CLANK that stops your motion right there. A simulator has to be able to replicate both. VR isn't up to the task. What's described ("your hand will keep going forward but the throttles won't") is not consistent with the expectations of a Level C flight simulator. (I don't watch the throttles when flying.) For a procedures trainer, then, sure.

In case it isn't clear, I fully agree that VR is not an adequate substitute for the physical reality of a simulator cockpit, but could be adapted to a VPT.  (have we been "arguing" when we actually agree?!)  :0

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Advertising During Eclipse
I imagine some companies will fly banners or sky-write during the eclipse.
Do all need to submit a plan and get it approved by the FAA?
Could a company actually be allowed to block the eclipse "a little" for a big
festival?
EDIT: The answer here is not enough.  It doesn't even talk about filing a flight plan for a specific date.  So, are you saying any registered pilot (with the waiver to fly banners or to sky-write) could just wake up that morning and decide to advertise? That seems too easy and dangerous.

If I owned a business that planned on skywriting in front of people's view of a once in a lifetime event I would fear for my safety.

Related

"So, are you saying any registered pilot (with the waiver to fly banners or to sky-write) could just wake up that morning and decide to advertise?" If you have the waiver and your flight plan is accepted, why not? "That seems too easy and dangerous." assessment based on?

All the same airspace rules which keep everyone safe apply as usual, including whether a flight plan is required. If the FAA feels that's not the case they would issue a TFR, as the linked question mentions.

"Whether a flight plan is required" is exactly what I asked and still don't know.  @Federico If you're saying a flight plan is required, please post an answer.  A few more details, like the FAA will make sure for safety that there aren't too many nearby flight plans, would be nice too.

about the flight plan requirement: https://aviation.stackexchange.com/questions/1612/when-is-a-flight-plan-required

Thanks everyone, please correct this answer if wrong:  No, no flight plan is required.  But, the pilot must of course be licensed and the aircraft must be registered (any towed banner would additionally need a FAA 91.311 waiver).  So yes, some guy (or many people, as long as they follow VFR) could spontaneously decide to go up and block the eclipse.  There are no special FAA rules imposed for the eclipse.  This seems to be a good opportunity for companies to get mass advertising (even without blocking the eclipse), so I'm curious to see if anyone takes advantage.


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Is there a reason to be worried about go-arounds as a passenger?
The last night I had an "interesting" experience with an aborted landing, which was a bit dramatic for my liking. Heavy fog and darkness didn't help either. Pilot told us that the reason for aborted landing is a problem with navigation instruments.
Today I checked the flightradar24 data, which I've attached for your pleasure, but it didn't ease my worries since according to site's data the first attempt missed the runway completely.

How accurate are these flightradar's data?
How common is for pilot to miss runway in heavy fog?
Is there a reason to be worried while flying 50 feet above meadows in heavy fog? Possibly with malfunctioning navigation equipment?

Note that FR24 data is completely inaccurate, so the the two attempts not lining up do not reflect the actual landing attempts.

According to the map, the second (successful) attempt also missed the runway. Doesn’t that answer your question about accuracy?

@chirlu Not really, it could be that Google Map is not aligned correctly

@MladenJanković, automotive navigation needs the map aligned accurately within couple of metres and since Google Map is good enough for that, it is aligned that accurately.

Aviation annals are full of the blood of unfortunates who's pilot ignored the go-around gremlin whispering in his ear.

A go-around is nothing to be worried about as a passenger.  What would be worrying is if the pilots elected not to go around when conditions warranted it.
The dark likely isn't an issue with this go around as approach lighting systems can become exceptionally bright when requested (to the point you'll need sunglasses to look at them if visibility is good).  What was likely a factor was fog.  Airplanes need a minimum amount of visibility to land and if conditions are marginal you won't really know until you fly the approach and either see the lights and land or do not see the lights and go around.  For normal ILS approaches you would initiate this go around at 200 ft above the airport elevation.  For very low visibility approaches this decision height may be 100 feet or less above the airport elevation.
The point at which you go around if you fail to see the runway environment is well defined and procedures ensure obstacle clearance so there is no danger as long as procedures are flown as published.
Another possibility for the go around could be the airplane in front of you failed to clear the runway speedily due to low visibility.  If the preceding aircraft was still occupying the runway then your airplane would have to go around.  There are a few other issues that could warrant missed approaches (e.g. windshear) but in all of these cases the go around is the safer option than proceeding with the landing.
Your pilots made the safe decision to go around and this is what you should take from the experience.  There is zero reason to be worried.

To address your added specific questions:

Flightradar24 flight track data is unreliable.  It is neither true radar positioning or necessarily true GPS positioning.  It is nice for zoomed out flight tracks but is not reliably suitable for the precision during landing/takeoff.  Do not use it as a basis for worrying that your approach was not properly aligned.
Extremely rarely if you mean not being aligned with the runway.  Uncommon if you just mean not seeing the approach lights but otherwise on course to land.  The navigation signals used in ILS approaches are broadcast down the runway centerline for the lateral guidance and from just next to the runway approximately 1000 feet past the beginning of the runway aimed 3 degrees up for the vertical guidance.  These signals are only usable in a cone that radiated outward from the runway.  Furthermore the navigation accuracy of the ILS receivers are regularly tested on the airplane.  Combined with monitoring by air traffic controllers you will not end up on a parallel course that does not intersect the runway.
The vertical data is also not reliable.  Your airplane approached on the glideslope and if it made it down to 50 feet this would have been just over the very beginning of the runway.  After initiating the go around it would climb (likely to between 2000 and 3000 feet) and attempt another ILS approach.  At no time is the airplane lingering at 50 feet above the ground anywhere but over the runway for a very short period of time.

The video below is an example of a very low visibility approach and you can see an example of how low the airplane can get before visual contact is made with the approach lights and runway.  At predetermined points based on the aircraft, airport and crew qualifications the airplane would initiate a go around if these things were not seen.  For standard ILS approaches this would be when the airplane announces "200 feet".  For category II ILS approaches, it would be at "100 feet".  In the video below a category III approach is shown which has an even lower altitude to initiate a missed approach.

@MladenJanković I've added some points to my answer to address the stuff added to your question.

Well, flightradar24 data is GPS if the aircraft has it. This one clearly didn't, though.

How accurate are these flightradar's data?

Flightradar24 relies often on ADS-B transmissions from the aircraft. Depending on the aircraft the accuracy of the transmitted ADS-B position  varies from very accurate to not accurate at all. This particular A319 belongs in the latter category; the source of the data was the Inertial Navigation System.

How common is for pilot to miss runway in heavy fog?

Quite common. Depending on the aircraft, the flight crew and airport navigation facilities the aircraft can land in zero visibility. Nikola Tesla airport has Instrument Landing System (ILS) CAT IIIb on runway 12 (where you landed) which means that the navigation equipment can support autolanding. In this case the decision height can be at the runway, the minimum visibility at the runway is 50 meters. At the moment there is no Notice to Airmen (NOTAM) published regarding maintenance of the system so I assume it is operable.
I don't know whether this particular aircraft and its crew are approved for ILS CAT IIIb operations.
If either the airport, the aircraft or the flight crew cannot support autoland, then the runway has to be visible at a predefined height. If the runway is not in sight at this height the flight crew will follow the missed approach procedure.

Is there a reason to be worried while flying 50 feet above meadows in heavy fog? Possibly with malfunctioning navigation
equipment?

No there is no reason to worry. This is normal operation, it happens everyday somewhere in the world. The aircraft is guided by two independent guidance systems that cross check each other. If there is a discrepancy between the two, an alarm sounds and the approach is aborted.
The navigation system on the ground is carefully monitored, if anything malfunctions the system will raise an alarm. The area around the runway and especially around the antennae of the navigation system is kept clear of obstructions when the airport is under Low Visibility Procedures (LVP)


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Is there a general radio frequency in Class G for broadcasting intentions?
Let’s say you’re in class G and doing some aerobatics. Is there any standard frequency you might use (perhaps MULTICOM on 122.9?) to communicate your intentions?
Assume you’re not using flight following and that you’re VFR.

USA class G or class E, say on the northern California coast.

I was taught that 123.3 is a common low altitude air-to-air frequency, but I'm, on the east coast; I don't know if there's something in the FAR/AIM about it.

@mah the only air-to-air frequencies listed by the AIM currently are 122.75 for airplanes and 123.025 for helicopters (section 4-1-11). And chances are no one will be using those unless they too are engaged in some sort of aerobatics or formation / group flying.

123.3 MHz is one of three frequencies the FCC/FAA reserve for "aviation support stations"; these are typically set up to coordinate pilot training, ballooning, gliding and certain other uses. This particular frequency, along with 123.5, can be used without a permit (the last one is 121.95 and requires advance approval) but use by pilots or mobile ground staff must not interfere with use of the channel by permanent ground stations and their contacts.

MULTICOM is used when operating in the vicinity of an airport that has no tower, no FSS, and no UNICOM so wouldn't be appropriate here.
There is an air-to-air frequency (122.75) but most pilots don't monitor it unless they have a specific reason to (i.e. someone else that they want to talk to).1
You can listen to the closest airport or approach control frequency, but that would only help if the other aircraft is on the same frequency.
In short, there isn't a good option for this, and in my opinion, unless you have a spotter watching for traffic in the area it would be pretty hard to do safely since it will be very hard for you to watch the area for traffic while doing aerobatics.  Other pilots may also have a hard time spotting you because you could be approaching from odd angles.
If you can get flight following, I would highly recommend it, but there is an even better option:
Practice in an Aerobatic Practice Area.  See Where Do Airshow Pilots Practice? for more specifics on this.  Not only would ATC and airports in the area know where you were, but a NOTAM is issued any time that it is active to warn pilots transitioning the area.

1 See Services Available To Pilots for more details on communication frequencies.

MULTICOM 122.9 may in fact be appropriate: the AIM 4-1-11 says it's for "Activities of a temporary, seasonal, emergency nature or search and rescue, as well as, airports with no tower, FSS, or UNICOM" (my emphasis). Aerobatic practice sounds like it would fall under the temporary activities?

@pondlife Well, the biggest problem is that I don't know of anyone that monitors it except when operating at an airport as described.

@Lnafziger That's true, I didn't know about that comment in the AIM until I checked, and there's often a difference between theory and practice. But having said that, telling people that "you can just ignore the AIM on this one, no one else does it anyway" just doesn't feel like good advice.

I'm not aware of any regulation or AIM entry that specifies what frequency to be on.
I use the UNICOM frequency of the nearest airport when I'm out looping because I consider it to be the frequency most likely to be used by traffic near me.
It also has a mild safety benefit -- if something on my plane were to break, I'd already be on the right frequency for the airport I'm about to inhabit.

Yep, in my opinion this would be the safest thing to do. Other aircraft are almost certainly either going to be on the nearest airport's CTAF or on an ATC frequency, so using the nearest CTAF is going to give you the best chance of having your transmissions heard by other planes nearby. (If you're near controlled airspace, you could also consider calling ATC and letting them know you'll be maneuvering in a certain area; even if you decline radar service and switch back to a CTAF, ATC will at least know you're there and can pass that on to anyone else they're talking to).

There was an FAA NOTAM released recently that required all pilots to monitor GUARD when feasible (i.e. when not actively using or monitoring another frequency). That's primarily for picking up transmissions from distressed aircraft and/or being able to receive and respond to a hail from ATC or an intercepting aircraft. If someone has to go that far to get your attention you're already very wrong.

As far as I can tell, according to the AIM 4-1-11 you should use 122.9, which is described as follows (my emphasis):

(MULTICOM FREQUENCY) Activities of a temporary, seasonal, emergency
  nature or search and rescue, as well as, airports with no tower, FSS,
  or UNICOM.

The words "as well as" indicate that it isn't just for use around airports, and aerobatic practice certainly sounds like a "temporary" activity. Of course, flight following would be preferable if it's available.


text:
Is it possible to use thrust reversers to taxi backwards?
Can commercial airliners theoretically taxi backwards using reverse thrust?
If this is possible, why isn't it common?
I can already imagine some safety reasons...

Very common on turboprops as the prop angle can be changed to produce reverse thrust a lot more efficiently than a jet thrust reverser!

I have seen it done with DC-9s many years ago.

@Cimbian Yes, this used to actually be very common with DC-9 class aircraft. I saw DC-9s to powerbacks all the time in the 90s. With fuel prices nowadays, though, it's no surprise that they almost universally opt for tugs instead now.

I've powered back King Airs, Metroliners, and 727s without difficulty when tugs were not quickly available. An acquaintance powered back a 747-200 when a tug couldn't be maneuvered into a position that it could do it. His is the only incidence of a 747 being powered back that I'm personally aware of.

This is called Powerback, most aircraft can do it, but it is not done very often.
In a jet aircraft, the three main problems are:

Reverse thrust tends to throw a lot of debris into the air because the exhaust is deflected to the sides and up and down too. This debris can damage the engine itself, other things on and around the aircraft or injure someone on the ramp. It is less of a problem for aircraft with tail-mounted engines (so DC-9s often used powerback), but that engine configuration is not used as much any more in transport aircraft, as it is less aerodynamically efficient.
A related problem is that the compressor might suck in its own hot exhaust gases. This might lead to temperature-induced damages in the last (high-pressure) stages of the compressor and health issues for passengers, since the air conditioning system works with bleed air from the compressor.
Reverse thrust is rather inefficient on jet engines, so it uses a lot of fuel.
As Casey mentioned in the comment below, the pilot has to be careful using the brakes during powerback: the main wheels are very close behind the centre of gravity, so harder braking can lift the nose wheel, causing the pilot to lose control and even causing damage to the tail if it hits the ground (“tail-tipping”).

So a tug is both cheaper and safer.
In propeller aircraft, reverse is more efficient and does not throw up as much debris, so it is sometimes used. But a tug is still preferred because transport aircraft don't have any rear visibility, so the pilots can't see where they are taxiing. With tug the driver can see behind the aircraft, and a ground marshaller walks along side each wingtip with an intercom  connected to the plane.
Lastly, since aircraft are able to turn almost on the spot using differential braking and thrust and 180-degree nosewheel steering, at airports with few facilities there is generally enough space around the aircraft to permit the pilot to reverse out easily.

I didn't think about the fuel consumption. Nice answer, thanks!

@AndréStannek wheels are aft of CG, so braking will create a nose-up torque and sit the airplane on its tail. Standard procedure during powerbacks is "feet on the floor" to avoid inadvertent brake application.

Isn't the prevention of compressor stall also another reason that powerbacks aren't used in most aircraft? This is also one of the reasons that reverse thrust has to be cancelled relatively early in the landing rollout, too, right (along with FOD prevention?)

@reirab: I don't know. I always only heard the FOD argument, but I don't know enough to claim it is not true.

Powerbacks can also create projectile hazards on the ramp...

It can be done, in fact the DC-9 and MD-80 aircraft are approved for backing up using reverse thrust. It is called "powerback".
It is rarely used since it is quite fuel consuming, noisy and increases the risk of sucking up debris near the gate area causing damage to the engines.
Here's a video of an MD-80 backing up.

Those reversers deploying look like something out of Transformers!

@DavidRicherby would you believe that it's only 2 links and an actuator to get that movement?

Wow, seeing a plane back up be itself just seems wrong :-D Thanks!

Wow that powerback is NOISY!

@DavidRicherby those are known as target/bucket reverse thrusters: http://en.wikipedia.org/wiki/Thrust_reversal#Target_type

I'd like some reference for the high fuel consumption argument.  Sure the engines have to power up substantially to make sufficient reverse thrust (as the buckets that direct the exhaust are not perfect at creating reverse thrust) but in the context of fuel burned during a flight, the 30-40 seconds of reverse thrust doesn't seem like it would be a significant factor in the overall fuel planning.  Correct me if I am wrong!

@SkipMiller It is a trade-off between fuel consumption (and perhaps maintenance costs) for powerback, or labor costs and equipment costs for pushback. The 30-40 seconds may not be much compared to the total fuel costs but it may still be more expensive than a lowly paid pushback truck driver for 5 minutes. It is a beancounters decision in the end.

You have to have an Opspec to do it though. OPSPEC C065, POWERBACK OPERATIONS WITH AIRPLANES. A. C065 authorizes the use of powerplant reversing systems for rearward taxi operations...

Dang, that MD-80 backing up was pretty neat!

AAL, which has a large MD80 fleet, used to powerback from gates. They switched to tugs at their hubs rather quickly when oil prices doubled almost overnight, but they sometimes still do it at non-hub airports, when not needing to wait on a tug would likely save enough time/money to justify the wasted fuel.

Can commercial airliners theoretically taxi backwards using reverse thrust? - After an airshow at RNAS Yeovilton, Somerset in the 1980s a British Airways Concorde found it could not taxi out for departure as it had been parked too close to an adjacent hangar (and didn't have the required turning circle)... there was no suitable towing gear on site, or within easy reach... so after a discussion with his engineers the pilot decided to reverse taxi a short distance using reverse thrust.... the centre of gravity was adjusted by transfering fuel to the forward fuel tanks, fingers were crossed, and... it worked!  (I was there to see it, working in the control tower)


text:
Are GA airports typically accessible 24/7?
Is access to GA airports 24/7?
I always thought it was.   Every airport I've visited had gate-codes to open gates after hours.
But I recently started planning a flight to Salina, KS (KSLN), and the local FBO told me field access shuts down entirely at 10p.m.
As a transient flight, I'd have no way back to my plane after that.  I'm skeptical, as I believed airport access was 24/7.
Am I just completely mistaken in that belief?

According to the AOPA directory, KSLN is under security directive SD-8G, i.e. the TSA limits access to the apron. I guess (but don't know) that you need an escort to/from your aircraft, so if no authorized person is available then there's no access. Otherwise, access times are totally up to the local airport management.

Related: "Airport Compliance Manual"  although it seems very much devoid of information regarding access (even though it has a section called "Airport Noise and Access Restrictions).

The general answer is that the airport manager is in charge of ground access (unless the TSA is involved; see below) and individual FBOs can open or not whenever they like. There are no standard federal rules for the same reason that there aren't any for when gas stations should open and close: they're private (or state-owned) businesses.
So before going somewhere new it's always good to call and ask about opening hours, access etc. If there are multiple FBOs at the field, you can call them all: one might be open 24 hours a day even if another one isn't.
One specific point about Salina is that the TSA may be limiting access. The AOPA airport directory says that KSLN is under security directive SD-8G, i.e. the TSA limits access to the apron. That means you might need an authorized escort - most likely someone from the FBO - to and from your aircraft. If the FBO shuts at night there may be no one there, but FBOs in general are usually good at making special arrangements if needed.
And although your question seems to be about ground access, I guess it's worth stating the obvious: airports are accessible 24x7 from the air. Nothing stops you landing at 3am, filling up at the self-service pumps and then taking off again. But watch out for local practices like voluntary noise abatement curfews, e.g. at KPDK:

Pilots are encouraged not to fly between the hours of 11 p.m. and 6
  a.m. Due to FAA Regulations, PDK cannot implement a mandatory curfew
  at this time.

It's still legal to land or depart there during curfew hours, but you'll really annoy the airport management, FBOs, and local pilots who have to deal with the complaints from local residents.

I guess ultimately I confused the ability to land and takeoff there 24/7 with the ability to have access to the field 24/7.  It doesn't seem terribly productive to have landing/departing rights if you can't actually leave the field.    Sadly, it seems I will have to leave my friend's wedding early to make the airports 10p.m. curfew.

@abelenky I wouldn't give up just yet, I've never heard of any airport denying a pilot access to his aircraft because "we're closed". Did they explicitly say that pilots cannot get to their aircraft after 10pm? And did you call the airport manager as well as the FBO?

Very often, FBO's will open after hours for a flight which need to depart (for a fee, of course).  Just call them and ask about after hours access!

I emailed the Airport Manager, who referred me back to the FBO.
The FBO will let me back in, for a $100 after-hours fee, (which is a lot relative to the rest of the flight which is just a short C172 hop)

How high are the fences?

This will vary from field to field and I do not think there is a regulation on the matter. You may want to call the airport manager if there is one that is not at the FBO. If there are multiple FBO's on the field try another one, they may have 24/7 access.

Define "accessible". If you need ground access to your aircraft, that depends on gate code / FBO availability (or whether the FBO is willing to give you access through a code-locked side door).
If you mean accessible for arrivals and departures, there will usually be a note in the Chart Supplement (formerly the AF/D) as to what the restrictions (if any) are. For example, KSNA (Orange County in SoCal) has a restriction of "WHEN ATCT CLSD NO LCL TRNG OR TOUCH & GO OPNS."


text:
How accurate must fuel gauges be for VFR flight?
All Private Pilots should know the acronym TOMATO FLAMES for the Minimum Equipment List (MEL).
On that list, F means Fuel Gauges.But what exactly is required of fuel gauges?
Yesterday, my rental plane came with a note that the fuel gauges were swinging wildly, and bouncing to zero occasionally.   I planned a 90 minute flight, starting with full fuel, so there was no real risk of running out of gas.I decided the fuel gauges probably didn't meet the spirit of TOMATO FLAMES, but I admit that I don't know what the exact requirement is.
How accurately do the gauges have to work to be legal?

The indicator must be calibrated to read zero when that tank is actually empty of all usable fuel (in level flight).  Bouncing is fine as long as they do not bounce when the tank is actually empty.
See regs:

23.1337
91.205

Wow, that is a pretty low bar to clear!!  Thank you for the details.

@abelenky The expectation of the pilot, of course, is to land the plane before the gauge reads empty and to not take off if it does read empty, presumably even if you know there's fuel in the tank.  Having fuel when you think you don't is not going to put you in danger, it's being empty when you think there's fuel that's the problem.

As long as it's bouncing you're okay! Sounds like the fuel gauges on tugs. It was halfway between the high and low of the bouncing needle

A fuel indicator that always reads zero would satisfy this

That is a common misconception.

@SteveKuo Nope. Then the system would not "indicate to the flightcrew members the quantity of usable fuel".

The only legal requirement of fuel gauges on normal and transport category airplanes is that they read empty when the fuel tanks are empty (how convenient!).  In general, for normal category aircraft, they make use a float type sensor in the tank itself.  These floats can drive mechanical linkages attached to fuel gauges in the cockpit or actual rheostats which control solenoids which drive the cockpit fuel gauges.

Some airplanes eg J-3 Cub, Ercoupe, etc. make use of a fuel tank just forward of the pilot which utilizes a float driving a sight stick.

Large transport category aircraft use Fuel Quantity Management Systems which make use of capacitive measuring systems.  Basically a system which can sense the fuel quantity using the electric capacitance of air and fuel as a means to sense how much fuel is inside of a tank.  These tend to be considerably more accurate than mechanical gauges.

To the best of my knowledge most fuel quantity sensing systems are proprietary to the OEM with little input from the FAA provided they comply with existing FARs.

99% of this answer is not relevant to the posted question


text:
Is Part 91 Instrument currency for SIC similar to Part 135 requirements?
Part 135 instrument currency in a jet aircraft requiring two pilots, requires both pilots to be instrument current. But how about the same situation for a Part 91 flight? Does the SIC need to have his 6 approaches with tracking, intercepting, holding etc... in the last 6 months?

Only if you fall under the Fractional rules (Subpart K).  Otherwise SIC currency requirements for turbojet airplanes are specified in 14 CFR 91.531, which requires you to be current via 14 CFR 61.55 and has no instrument currency requirement (although you do need your instrument rating to fly under IFR).


text:
Why do only some aircraft require tail anti-icing?
Why do some aircraft require anti-icing on the tail while others (Dassault Falcon Jets, Boeing 737, 747, etc.) don't?

Some airplanes don't need the horizontal stabilizer to have anti-ice because it was shown during flight testing that there were no adverse effects from having ice on the tail.
This is mainly because the horizontal stabilizer/elevator has been designed so that even with a degradation of lift due to the ice, it still produces a sufficient tail down force to maintain control, even when approaching the stall speed.
The airplanes that do have anti-ice usually either have smaller horizontal stabilizers or (on some older designs) couldn't get the FAA to sign off on a design without it.  Some designs are also more prone to collecting ice (based mainly on the radius of the leading edge) so must demonstrate sufficient handling qualities with even more ice, which may not be possible.

I would expand a little bit more the answer. Firstly you are talking about anti-icing systems. Actually there are 2 kind of devices that provide protection agaisnt icing, from one side the anti-icing systems, like heaters, and de-icing systems, once iced is created the systems destroys it. For example, F17 used de-icing boots on the horizontal plane, so it was not using anti-icing. ATR72 is another example of de-icing systems.
However, I think your question is actually why some airplanes are not using ice protection systems (both de-ice and anti-ice) on horizontal planes.
The answer is clear: because is cheaper to have a bigger tail.
Let's recall what's the horizontal plane is doing, its function is basically providing balance to the airplane and supporting maniouvres in flight, like for example take-off, recover from a failed landing...
When designing an horizontal tail plane you need to consider all potential maniouvres that the plane needs to answer to. One of those conditions need to be possible with the presence of ice or having a system avoiding having ice.
So, to provide the same performances, either you have a system that avoids having ice or very few or you have a bigger plane. On top you have other conditions which are dimensioning the plane, like flutter or other maniouvres.
So...

Either was lighter having a bigger plane that operates safely with ice than a smaller plane with ice protecution
Or there was another condition in the design that was already making the plane big enough to operate safely with ice, making the ice protection system not mandatory and so not needed weight.

To be honest, I can't tell you for each airplane which was the dimensioning condition that eliminated the need of ice protection.


text:
How much snow can cover a runway while still allowing commercial airliners to land safely?
In regards to this question I thought about how much snow is shown on the runway and the exits in the movie.
So normally a commercial airport will have a fleet of snow plowers or heaters to keep runways, taxiways and aprons mostly clear of snow. But what is the safety limit on when the airport would have to close down due to too much snow coverage on the runway (not due to heavy snowfall, but due to the runway being covered completely in snow).
I think once the markers are no longer visible you would have a severe problem navigating on the ground or once snow piles up so high that the engines would take in a lot of snow it might also be a problem. But what are the actual limits during which one could safely land an aircraft on a snowy runway? And has it ever happend that someone landed on a runway covered in snow anyways?

In the end it will be up to the pilot to decide. I'm not sure there are any defined/legal limits.

Most companies have snow policies of one sort or another. Corporate ops generally do as well.

There is no set legal limit (ultimately it is up to the PIC to decide) but condition reports are available to pilots at many airports to assist in assessing takeoff and landing performance.   The FAA has recently created a standardized way of reporting runway conditions through their TALPA (takeoff and landing performance assessment) initiative.
TALPA is an improved way of assessing runway conditions, based on contaminant type
and depth, which provides an aircraft operator effective means to anticipate airplane
braking performance.
The RCAM (shown below) is the tool airport operators use to report a runway surface assessment when contaminants such as water, ice, snow, slush, and others are present on runways, taxiways, and aprons.   To find an updated RCAM look in Chapter 5 of Advisory Circular 150/5200-30, Airport Field
Condition Assessments and Winter Operation Safety.
The RwyCC is the runway condition code -- as you can see a code of 0 yields minimal to non-existant braking and/or directional control.

I cannot offer definitive criteria, however, I have seen a 737 land at a regional airport with 3 to 4" of dry snow on the ground. I can also remember a 727 departing pre-dawn where the runway had about 5" of light snow.  I later talked with a 727 driver who said that under those circumstances, there was little concern with loading the gear with slush, and the 727 had good takeoff performance. Then again, much of my flying has been in the northern US, and arctic regions.
Virgin snow is relatively easy, but unevenly packed snow, or frozen slush are different risks.
If we're talking turboprops, then the snow ends up getting deeper and wetter.  But the term was "airliner."


text:
What is this radar-like thing?

A friend of mine sent me this picture, which Is somewhere in Miami. But I don't seem to recognize it

The blue projections on top are lights, I believe.

what a big old thing!   Any UX designer would say they wish they set the wings "back" a bit so it is more definitively "icon-like" !

I agree , even a foot back would make it look more like an aircraft.

It is a wind tee; it serves the same function as a wind sock. It is designed to resemble an airplane from the air so that pilots overflying the field can more easily determine in which direction they should land. The top of the "T" is the front of the airplane and represents the direction in which an airplane should land.
When the wind hits the vertical tail surface it weathervanes. The tail points in the direction that the wind is blowing.
Unlike the wind sock, the wind tee does not provide wind speed information, only wind and landing direction.

Does it show how strong the wind is, or only the direction?

Just the direction.

It's a lighted 'wind tee'. this website shows various wind direction indicators

It's a weathervaning landing T, one of the standard landing direction indicators defined by ICAO, complete with lights for use at night. That sign, together with others, is often placed in the airfield's signal square, such as this one:

(source: Touchdown Point)
The tail of the pivoting T, thank to its vertical fin, always points downwind, showing to overflying pilots the landing direction as the one that would make you read the symbol as a 'T'. Other, simpler installations have a T lying on the ground that is rearranged manually to switch between the two landing directions of the runway, if present.
It is unclear what the symbol itself is supposed to represent, if anything. The original Annex 14 of ICAO's Convention on International Civil Aviation (p. 42) -  first published in 1951 - only specifies shape, size and color of the landing T; no rationale is given.


text:
Has the financial situation of an airline ever negatively impacted safety?
Has a bad financial situation, or simply being greedy, impacted negatively the safety of an airline or even caused lethal accidents?
Notice that this is not necessarily a question about low-budget airlines like  Ryanair and EasyJet, JetBlue, or Spirit. Low ticket price does not imply the company is in a tight financial situation. No frills does not imply cutting costs at the wrong end.
Although regulations are the same for all, there are certain scenarios where this seems to matter. No airline will be allowed to skimp on safety issues, but wouldn't having less resources imply:

Hiring cheaper pilots, which fulfill minimal requirements, and not much else.
Flying routes that are open, but less safe, like Ukraine until recently.
Putting more pressure on pilots to fly against their judgment.

@Federico: that's absolutely not the case. This is not about low-cost, it's about airlines with financial woes. Low cost/no frill airlines could be in a good financial shape.

ah, I missed a "not" in the first remark. anyway this remains a question either too broad (too many airlines, each with its own market strategy) or opinon based.

It's a valid question, and I think that when budgets are tight at an airline, they will do the same scrimping that people do when the household budget is tight. I can see inspections being contracted with companies that may do a light inspection for less money and overlook items, etc. However, I agree that this is far too broad to be adequately answered at Aviation.SE.

What if the question was rephrased to ask when financial situation has impacted safety in the past? This would allow the references provided for answers to be based on NTSB (or other investigating entity) opinion rather than the opinion of the author. ASRS reports might also make good references if this change were made.

That certainly sounds reasonable, @RyanBurnette, do you know of any specific NTSB results that would be applicable, or are you just speculating? I'd go for the edit & see if it gets reopened.

West Caribbean Airways 708 comes to mind. In this case the pilot was working a second job as a bartender. The airline was in major financial crisis at the time. https://en.wikipedia.org/wiki/West_Caribbean_Airways_Flight_708 I'm sure you can find more cases where finances impacted safety and the report states that very clearly.

I read an article a while back about pressures commercial/charter/airline pilots can face. There is often pressure from the operator to push weight and balance, fuel reserve and other operational limits in order to conduct a flight and make a profit. If I could find it I'd post the link.

Interesting read on WCA708, but the crash seems to be due to the pilots mishandling a stall, not any sort of failure due to poor operating conditions due to financial lack. The financial situation of WCA came under criticism, but wasn't directly implicated. Of course, the pilot may have been exhausted from bar-tending prior to showing up at the airport for the flight, but that's not specifically mentioned in Wiki. This might be a very difficult question to answer, but it still seems like a reasonable one to ask.

Another example: Manx2 Flight 7100.

Pan American World Airways declared bankruptcy in January 1991. In the year previously they were doing everything they could to to avoid that. To help their cash flow, Pan Am sold five of their 747s for cash to Evergreen International Airlines. The deal was that EIA would immediately lease the five back to Pan Am, which they did. Unfortunately, Pan Am soon defaulted on the lease payments and EIA began repossessing the aircraft.
EIA gave Pan Am fair warning on the repossession of the first aircraft, at which time Pan Am swapped out good components for crap components before surrendering the aircraft. Not to be taken again, when EIA repossessed the second aircraft, they did so without warning at the gate when the aircraft arrived in JFK from Europe.
I was the f.o. on the crew that was sent to get the aircraft. We were in the cockpit setting up when one of our maintenance guys came in and said that they had just received orders to boroscope the engines, which they did while we waited in the cockpit.
After an hour or so, one of our JFK managers came into the cockpit and told us that we wouldn't be going anywhere, that the airplane was not flyable as three of the four engines were significantly below required tolerances.
So, if flying a plane load of passengers across the North Atlantic with three out of four engines below required minimum maintenance standards constitutes a negative impact on safety, then, yes, a bad financial situation did indeed, at least once, cause such.

I would love to sit down with you over a drink to discuss your career!

Jeez. That makes two of us, @Ben. Get a booth!

I'd imagine we could rent a university lecture hall and fill it up with Terry fans from here!

@Ben Some sites run chat sessions with a specific user: A date and time is scheduled and users can ask anything, the chat log is then cleaned and published as a blog post. You can consider something like this :-)

To discuss some of your points directly,

hiring cheaper pilots, which fulfill minimal requirements, and not
  much else

Patrick Smith touches on this in his book Cockpit Confidential (in regards to cheaper airlines but the same idea applies) and basically comes to the conclusion that PIC time has little to do with anything and the statistical differences are small. He mainly points to the fact that the deadliest accident in aviation the Tenerife Airport Disaster occurred not only on the ground but the pilot at the controls was the airlines chief 747 type instructor Jacob Veldhuyzen van Zanten and extremely experienced both in type and as a pilot overall.

flying routes that are open, but less safe, like Ukrania until
  recently

The most unsafe thing in this case seems to be commercial planes either intentionally or accidentally being shot down, you can find a list here.

putting more pressure on pilots to fly anyway

The FAA regulates the hours a pilot must rest for (and an airline cant short cut this) as for departure or not the end call (as far as I know) lies with the pilot. Your takeaway point here is mainly about management putting pressure on employees to do things that may not be safe or may be out of the machines capability. This is not all that dissimilar what some say the cause of the Challenger disaster were.
You may be able to argue that an airline strapped for cash may put their pilots in poorer accommodations which in turn could lead to a lesser nights sleep and degraded performance but I do not know of that causing any accidents. It did take many years for the FAA to change required rest regulations and you can see some of them here, this was considered a big win for pilots.


text:
Why are military and civil aircraft landing gear generally so different?
Why do military aircraft have a lower landing gear than civilian? I have in mind the difference between a Lockheed C-130 Hercules and a Boeing or Airbus.
The first possibility that jumps to mind is that one is more comfortable than the other, which is more robust. But would an aircraft designer reduce robustness to provide more comfort?

What do you mean by "lower landing gear"?

@casey: I mean shorter, the distance aircraft to the floor is shorter.

That just ain't true. ATR-42 or BAe146/AvroRJ have similar gear to C-130, not to mention Lockheed L-100.

This really comes down to high wing vs low wing. Many other military aircraft use "longer" landing gear

@JanHudec: One kind of expects that the L-100 would have similar landing gear to the C-130, given that it's literally the civilian version of the C-130.

There is no difference between civil and military aircraft. Not the least because the civil and military transport aircraft are actually the same models. Many military planes have civil variants like Lockheed C-130/L-100, other military planes are derived from civil ones, e.g. C-135 is B707.
The difference depends on where the wings and engines are placed. All the aircraft actually have similar clearance from the ground. Just on some the lowest point is fuselage, on others it's the engines hanging down from the wings and on propeller driven ones it is often the propellers.
Cargo planes (like C-130, C-5, An-124 or An-224) are often built with high wings, so the fuselage is lower and therefore easier to load. This is also concern for regional airliners as they are designed to carry their own stairs. That's why BAe146, ATR-42 or Dash-8 are high-wing and other like Bombardier CRJ series have tail-mounted engines so they can also be lower. On the other hand low or mid-wing designs with wing-mounted engines have better aerodynamic properties, which is why that construction prevails for large airlines.

and don't forget that the 146 was intended as a military transport before being marketed as well as a civil airliner and cargo aircraft.

Easier to Load
As stated earlier, this is your primary reason. You have to be able to load in cargo from the ground. If you want to get troops and vehicles in fast to somewhere with minimum equipment, you want to be able to roll them of the aircraft, and the lower the aircraft, the easier this is done.
Other considerations:
Shorter gear is stronger
The longer you make the gear, the more it will bend. Bad news if you're trying to land on rough ground.
More Cargo space
The shorter you make the gear, the less trouble you have finding space for it inside the aircraft, where you want to maximise the amount of space you have for cargo, especially for vehicles and the like. The anatomy of a cargo aircraft is that they are typically hollow inside, unlike passenger aircraft where they fold up into the center section.
Landing gear is heavy
Landing gear is very heavy, since its got to support the weight of the entire aircraft above. The shorter you make it- the lighter the aircraft gets.
Better use of ground effect
The low gear may bring the aircraft a little bit closer to the ground, maximising ground effect and lowering your landing speed, again very ideal for rough terrain.

Note: These don't only apply to military aircraft, but to a lot of commericial ones as well, since the benefits are the same. Look at the Dash-8, ATR 42/72, BAe 146 along with the rear-engined planes, and they all try to do this for these reasons, although ease of cargo loading might not come first on the list.

I am not sure about the rough ground. On rough ground, you need big shock absorbers.

Countering that: military aircraft that need to operate from unprepared strips may want taller landing gear so they have more ground clearance when operating on rough ground.

@jwenting  http://www.youtube.com/watch?v=zj0a0wtGQKo shows that you can come a pretty good way with short gear and if you're stuck in the mud you wouldn't want them to be longer than necessary.

I think this answer ignores the actual primary reason: wing and engine location. High-wing designs don't need as much ground clearance to keep from dragging engines, wingtips, or flaps on the ground as low-wing designs. This is especially true in the case of low-wing designs with large wing-mounted engines. This is specifically why the 737-900ER had to have longer landing gear than previous 737s, so that the new, larger engines would fit under the wing and still have enough ground clearance. As far as ground effect is concerned, I would assume that low-wing designs will win there.

Also, the 'shorter gear is strong' part doesn't hold up very well, either. Yes, it will resist bending moments better, but it gives you less travel on the shocks, which means higher instantaneous forces are experienced during touchdown (same work has to be done over less distance.)

For the aircraft you use as examples the aircraft height above the ground (landing gear height) is mostly a function of use-case and clearances.
For the Boeing and Airbus airplanes you mention, the gear need be long to accommodate the engines hanging under the wings as well as the minimum bank angle needed to avoid wing strikes.
On the other hand, the C-130 has the engines mounted up on a high wing and the aircraft needs to handle big slow heavy things like tanks being driven into the cargo.  The need of the tanks specifically probably mandates the interior floor be pretty close to the ground, or you might need extremely long ramps to for the tank to ascend (and these would pose their own problems).

Latter case is solved in some transport aircraft by having the aircraft "kneel" after landing by use of hydraulic pistons in the landing gear. Effectively you're forcing the shock absorbers to fully compress.

The tank-specific parts of this answer are actually quite a way off. Tanks are designed to cope with very rough terrain so a short, steep ramp wouldn't be a problem for them (though fitting a large vehicle in at an angle could be an issue). But this is moot anyway because a C-130 can only carry about 20t of cargo, which isn't close to being enough for an M2 Bradley (30t) , let alone a full battle tank (60t for an M1 Abrams). But, still, a low cargo bed is useful for heavy cargo, as you say.

C-130 Addendum
Purpose-designed for cargo - rather novel at the time back then, perhaps. Look at WWII films of the C-47 or Berlin Airlift film of C-54's. How the heck do they get anything in there?
Designed around a "standard truck bed height" floor and a "standard railroad box car" dimensions - 10' x 10'.
The short landing gear derives from the basic design goals. Further, the wheels are in tandem - one behind the other vice side by side. And the tires are fat. These aid operating out of unimproved/dirt fields. The leading tire "plows the way" so to speak for the rear tire. Fat tires lowers the "per square foot" weight on the tires.

The longer you make the gear, the more it will bend. Bad news if you're trying to land on rough ground.

Ironically the C-130 landing gear is wimpy when it comes to side loads. It does not have adequate side-bracing. Straight ahead, we can pretty much plop that puppy on the ground relatively aggressively. But, during taxi, stopping while still in a turn is a write-up and requires maintenance inspection prior to flight.

The first possibility that jumps to mind is that one is more comfortable than the other,

The C-130 is designed for utility, by masochists. It is painfully loud, too cold and too hot at the same time, vibrates like the inside of a jack hammer; and if the the prop synchro-phaser ever fails will drive you insane in short order.

The biggest factor in landing gear length is going to be how the engine(s) is/are mounted and ensuring they have clearance.  The C-130 and turbo-prop commuter airliners (Embraer and Bombardier) have the engines in line with the bottom of the wing which is attached to the top of the fuselage.  Large airliners (Airbus and Boeing) have engines slung well under the wings which attach near the bottom of the fuselage.  The turbo props get a lot of clearance from the height of the fuselage while the jets rely on longer gear legs.


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Can an airliner provide ground level cabin pressure, temperature, humidity etc?
Apart from the cost angle, which seems to be the biggest factor in airlines, is it technically possible to bring cabin atmosphere (includes pressure, temperature, humidity) etc. at par with ground level, at the cruising altitude of most airliners (say 35000 ft).
I just want to understand what stops airliners from providing this kind of comfort to the passengers. I am assuming there might be issues of hull integrity in order to maintain higher cabin pressure etc.

Just for clarification, all of this is outside of the control of the airline. It's all a matter of aircraft design decisions made by the manufacturer, not operational decisions on the part of the airline. The airlines just follow the operational guidelines set by the manufacturers.

"...is it technically possible to bring cabin atmosphere...at par with ground level" Yes. "what stops airliners from providing this kind of comfort to the passengers" Cost.

As the customer of the airframe manufacturers, it's not really outside of airliner's control - if airliner passengers demanded sea level pressurization from the airlines, the airlines in turn would demand it from manufacturers (which would, of course, drive up costs). And once one manufacturer offered it, they all would have to in order to remain competitive. It's just that the status quo is good enough for most passengers so no one is demanding greater pressurization (though Boeing may have started a trend toward greater pressurization with the Dreamliner's 6000ft level).

The Airbus A380 has a cabin altitude of 4,990 feet (1,520 m), lesser than the Dreamliner , 6,000 feet (1,800 m)

The SyberJet SJ30 is one plane which offers 'sea level cabin' (zero cabin altitude) up to 41,000 ft (due to its 12 psi differential pressure)

See also "cabin pressure for private jets" , a question on the en.Wikipedia reference desk: https://en.wikipedia.org/wiki/Wikipedia:Reference_desk/Science#cabin_pressure_for_private_jets

I'll touch on the humidity a bit.
This is actually a big issue for planes; they tried to make it a bit better on the Dreamliner and were able to bump it up a bit

Humidity is improved, yet still dry as the desert. Humidity levels on the Dreamliner are 10%-15% — better than 7% on other planes on long trips

source
but it's still a big issue. There are two main reasons they can't really increase it that much.
First off, water is very heavy. Since planes pump in dry exterior air they would have to humidify it which means they need to carry enough water to do so. This weight would greatly affect the useful load of the plane. Now you could try to recycle some of the ambient moisture, but on a large scale that takes power, which takes fuel, which comes back to the weight issue.
The other issue (although not as often thought about) is the longevity of the airframe. The extremely dry conditions that airliners see for most of their time actually helps to prevent corrosion on the airframe as well as rust on any rustable parts. By pumping moisture into the airframe you will run the risk of it causing real damage to the metal components.
The temperature is a simple energy problem. To keep a plane hot you need to heat it faster than the exterior temp is cooling it (or evenly to keep it a constant temp). Insulation can help with this but you are still going to see some cooling. On a rudimentary level you can apply Newton's law of cooling here and keep in mind the exterior temperature is in the -40 (and below) area (according to today's charts).
On to the pressure,
This, of course, is a big one, but, in reality, it's not a big deal. While it may seem a bit irritating, 8000ft atmosphere equivalent is still more than breathable. The FAA does not require continuous O2 until 14,000 ft for unpressurized planes, so an 8000 ft equivalent cabin is more than fine. Now from a comfort standpoint, your ears may pop and you may feel some discomfort, but it's more than safe.
From a purely technological standpoint we can do this, but from an engineering use case standpoint it's better to sacrifice some of the pressure to build a lighter, thinner plane. This saves on fuel and material costs down the line. There is great hope that carbon fiber may bring a change to all this and 8000 ft equivalency will be a thing of the past. Boeing has been pushing this with the 787 having only a 6000ft pressure level which they claim alleviates problems. I have not yet flown one so I don't know first hand how much better it really is. Then again I fly unpressurized stuff often and have become somewhat acclimated to it.

Cabin "heaters" in jets don't heat, they cool.  The cabin air is taken from the engine intakes, where compression heating raises the temperature to 100C or above; it is then cooled to a reasonable level by running it through a heat exchanger.

@Mark: The air needs to be cooled, but for the cabin itself it is still heating. It does indicate that it is a non-issue though since just compressing the outside air makes it hot enough for the purpose.

Temperature is not so much an energy problem (compression for pressurization heats the air more than enough) as it is a problem of defining what the temperature should be. Usually 21-22°C is considered normal temperature and that is what the air conditioning is set to throughout the year. If there is 30°C outside before you board, it feels cold inside, but then you may land somewhere else where there is 10°C and then at least the shock is not as big. And on another day there can be -30°C and the 21°C will feel pleasantly warm.

I have flown the Dreamliner several times, and I can tell you it feels much more comfortable in terms of cabin pressure, even at altitudes in excess of 40,000 ft

The air in the cabin is usually taken from the engine, where it is highly compressed. So, doesn't compressed air usually contain higher amounts of moisture?

There is barely any moisture in the air at altitude. Compressing extremely dry air still leaves you with extremely dry air.

Weight and strength.

At 10700 metres (35000 feet), the ambient pressure is about 24.8 KPA (3.6 PSI).
The pressure inside the aircraft is about 75.8 KPA (11 PSI), assuming 8000 feet cabin altitude.
Imagine a cabin door approximately 2 metres by 1 metre (18 square feet).
The force on the door is about 3970 KGs (8750 pounds or 4.3 tons).
Imagine now that the cabin is pressurised to 100 KPA (14.5 PSI).
The force on the door is now 5830 KGs (12850 pounds or 6.4 tons)!

And that's just one door.  Now imagine the extra force for all of the other doors, the windows and the fuselage itself.  The additional force would be huge.  To build a fuselage that strong would be very heavy.
The temperature can be controlled no matter what the cabin altitude and most people can cruise at 8,000 feet with no significant health effects.  Therefore, it is just not necessary, or cost effective, to build an aircraft strong enough to fly with a cabin altitude of zero.

I agree with almost all of this answer, though I wouldn't go so far as to say that there are no ill effects. Maybe no significant health effects would be a better way to word that. Anyone who has spent 14-20 hours at cruise altitude flying halfway around the world can attest to the nasty feeling resulting from spending that long at such low humidity and pressure. This is discussed further in this question about the 787.

Where did those force numbers come from? I'm trying to replicate the calculation (as $F=A_{\text{door}}(P_{\text{in}}-P_{\text{out}})$), but I'm getting around 150 kN if the cabin is at ground level and 100 kN if the cabin is at 8000 feet (in lbs-force, that's 23,000 lbs-force at 8000 feet and 34,000 at ground level).

@cpast Good point. Just as a quick sanity check, (14.5 - 3.6) / (11 - 3.6) = 1.473, so the force should 'only' be 47% higher using the pressures listed in the answer. Also, unlike lbs, kg is not a unit of force.

@reirabOK, looks like my "count on your fingers" foo is not so good.  I'll come back later and redo the maths.

@Simon But isn't that assuming that the strength determining failure mode in an aircraft is the internal pressure? Is it? i.e. When you do the thickness calculations for various failure modes, e.g. ground handling, buckling etc. is the skin thickness really set by the "pressure vessel" mode of failure?

@curious_cat  It is the determining factor, or more precisely, the pressurisation/de pressurisation cycles which apply repetitive stress to the fuselage. If the pressure were increased, the stress would be increased.

@Simon Understood. It's the fatigue cycles rating. Because it couldn't have been the skin thickness per se as even the current thickness is sufficient to take a much higher delta-P across it as a pressure vessel. Its the fatigue loading that's critical I assume & affected by the delta-P.

I would like to point at an aircraft SJ30-2, in which the cabin is pressurised to 12psi at 49,000 ft, but has actually been tested to 31.4psi, a world record achieved during an FAA pressurisation ultimate load test.

In addition to needing structural strength to hold in a greater amount
of pressure, the airframe will undergo repeated stress cycles due to
alternating conditions of equal pressure (on the ground) and
low outside pressure with high inside pressure (at cruise altitude).
Making the stress cycles deeper (by increasing the pressure difference
without changing airframe structure) can
cause more rapid failure of the fuselage.
Reducing the stress means increasing the material you put into the fuselage,
but almost surely not by just adding a few reinforcements and
(I think) not generally even by just making everything stronger.
To safely increase the pressure difference, I think you'd need to
substantially redesign all the parts of the airframe that hold in the
pressure (potentially down to individual panels and rivets),
and then sacrifice an airframe to run a lengthy test
of pressurization cycles to verify that it doesn't fail.
It seems reasonable to do such a thing for a completely new model
(such as the Dreamliner), where you already have to do that amount of design
and testing for some level of pressurization,
but I think you'd need a stronger incentive
to redesign an older model in that way, and retrofitting existing
aircraft would (I suspect) be prohibitively expensive.
TL;DR: with a completely new aircraft design, maybe (if you consider it
worth the cost); with existing airliners, no.

Some small aircraft manufacturers, like Gulfstream, claim to provide cabin pressurization equivalent to 4,000 ft at 40,000 ft

@Firee Sure, that would be another example like the Dreamliner (but more so), in which you can design a higher cabin pressurization into a new aircraft type. And I suppose there are quite a number of airports you can fly between where 4000 feet would be "at par with ground level".

No? You wanna know why?

Strength

Aluminium has lower tolerable stress than steel, while steel can keep its stress tolerance fairly well under repeated use.

Weight

There's a maximum pressure difference of 8 psi for aluminium, with few exceptions. You literally have approximately 4 tons of pressure for 2 square meters of aluminium with around 8 psi of pressure difference. And also people leave aircraft unpainted in the interior to save weight.

Corrosion

As I said about the unpainted interior. Aluminium will fatigue quicker if corroded. Meh, common sense.

Strict safety regulations

8000 ft is the maximum cabin pressure? I think that's too strict. Suggestions.
10000 ft. I mean it's easy to acclimatize to 10000 ft without real hazards or reducing comfort. Average blood oxygen saturation will be a hair less than 95% at 10000 ft. In contrast, it's usually 99% at sea level.
4000 m. Above 90% is OK for patients with COPD. At 4000 meters most people would have blood oxygen saturation around 90%.
5000 m. The standard of acclimatization without too harsh hazards. Might start to see some harsh hazards. So 5000 meters is the maximum cabin pressure most people can tolerate without harsh effects.
So guys, for 5000 meter cabin altitude to work, ban alcohol consumption in planes. At least it'll avoid the headaches.

It's not easy to see how your points relate to the question. I think your answer could be more helpful if you [edit] to explain a bit more: why does an unpainted interior make a difference to the cabin pressure? Why does the lower tolerable stress of steel matter? The second half of the answer doesn't seem to relate at all: the question asks if the pressure altitude could be lower, and you say it could safely be higher.

Welcome new user!  Stream of consciousness answers sometimes contain hidden gems, but, the unfortunate reality is a more conventional, structured approach to writing answers does, on the SO sites, lead to greater vibes.


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What would prevent the installation of windows on the roof of an airliner?
What prevents airline manufacturers from putting a strip of glass along the roof of the aircraft? It would be a good addition in terms of cabin view, light inside the cabin, etc.
Would it compromise the structural integrity too much?

Edit: The image is for representational purposes only. My question does not pertain to a installation similar to this image

I read somewhere dassault aviation do it on the falcon 5X. This may be a natural evolution of increasing windows size.

Ironically, the picture you posted is for a windowless aircraft concept, where screens replace windows completely. That seems like a much easier way to give passengers an all-around view than dealing with the structural issues of huge gaps in the fuselage.

That's all good and neat and all, but oh my god, I would wager way more people would experience vertiginous gastronomical expulsions than desired.

Even with highly reflective glass, it would get uncomfortably hot in the cabin.  The idea is to get sunburn on the beach after you arrive, not in a plane on the way there.  No flying greenhouses for me.

I know that aquariums hold back massive amounts of water pressure with glass, but that glass is massively thick. How in the world would you pressurize an all-glass hull like that with glass thin enough to get it off the ground, and how would it take all the stresses of take-off and landing?

@FreeMan Aquaria are also are withstanding pressure on the convex side of their arch/cylinder structures, as opposed to on the concave side.

vertiginous gastronomical expulsions - @CGCampbell, that is quality word smithing right there!

@Pondlife: Your observation is correct, hence I have put a disclaimer that the picture is representational. However, on the concept of screens, I wonder, if they are going to show the actual view outside or a rendered image. If its a real time image, I would worry about video lag, which might have serious repurcussions on the passengers, watching jittery images all around.

@FreeMan: The same way windows work in an aircraft

@Firee, until reading the link provided by Pondlife about windowless aircraft , I thought the intention was that the majority of the hull would be glass. That would be incredibly heavy as the glass would have to provide all the structural support for the entire aircraft. The windows on a standard airliner simply have to resist air pressure, not provide mechanical support.

Quite apart from structural engineering issues, most people just don't want to look out the windows.  I recall one trip a decade or so ago (before I gave up commercial flying).  Denver to Reno, at night, with a full moon over a cloud deck and whispy streamers of cloud reaching well above our altitude.  Honestly looked like one of those fantasy illustrations, only missing a few unicorns.  Only people that saw it were me and a kid a couple of rows ahead: everyone else was happily watching old TV game shows.

Sweet, windowless aircraft... to solve the "vertiginous gastronomical expulsions" issue, provide each seat with a VR headset instead - passengers prone to motion sickness can simply forego their use, and those who do use them would get a far more awesome perspective.

@jamesqf I am just the opposite, but I can probably count the total number of flights I have made in my life on two hands.  Not having a window seat is always distressing to me as I love to watch what's going on, and people who have a window seat and then decide they are going to shut the window (so I can't even get a peek out) are even more frustrating! (I just don't have the gall to ask such people if they would trade seats with me...) In line with this, I am probably in the vast minority of people who would be happy if the just side and bottom of the plane was mostly glass.

@Michael: Yes, or the cabin attendants who insist you shut your window so that people can see the movie :-(

@MartinJames: having flown gliders a lot, I can tell you that even on the hottest days of summer it is kind of cold already at about 2000m above ground. At commercial jet levels, I would be really surprised if the greenhouse effect wins over the -50C on the other side of the glass.

There's probably not a very interesting view out of the top of an airliner, unless something has gone horribly wrong...

@ManuH I thought of the same thing when I saw this, although they are calling it a skylight:  Dassault’s New Falcon 5X Feature An SPD-Smart Skylight

I thought this was about installing the Windows operating system on the roof of an aircraft. That'd be a little more challenging.

@MartinArgerami: That is an excellent insight... It would indeed be quite cold out there, else passengers would have been burnt already from the rays coming through the normal windows...

it's been done: https://en.wikipedia.org/wiki/Stratospheric_Observatory_for_Infrared_Astronomy#/media/File:SOFIA_with_open_telescope_doors.jpg

There are a few factors involved in this and you should check out this answer that touches on window size (same issue really). For what its worth the ceiling is prime real estate on an aircraft. Modern airliners have the over head baggage compartments there (which provide a remarkable amount of space)

The area is also used to run cabling (electronics mostly these days) fore and aft as well as power to the reading lights etc. However it should be noted that these things could most likely be routed along the floor just as well.
The real issue comes back to windows being a weak point and glass being heavy. When it comes to planes you want them to be as light as can be so excess windows will only reduce useful load and increase potential failure points. On top of all that you will have more ambient light in the plane which may bother those trying to sleep on late night flights.
If you want a better view fly an unpressurized plane!

While on the topic of small bubble canopy planes, anyone that flies GA planes (especially like the ones shown above) with out air conditioning will tell you how hot it gets on the ground (and even at lower altitudes flying) in the summer. The canopies make the inside of plane behave like a green house and really heat up. This would make the inside of a jumbo jet really bake in the summer while taxiing (which can be a long process at big airports).

For a second (while still reading the text above) I though the yellow plane was a model that was somehow balanced on the windshield of the white plane...

I don't know about GA, but a C-130 gets ridiculously hot on the ramp, despite having basically no windows in the cargo compartment. Running the A/C helps a bit, but there's only so much it can do.

If glass is heavy, i am sure other composites could be used... If the windshield glass in front of the pilots can take the pressure, i am sure glass on the roof can handle it too.. Also, it need not be a huge area of glass, a thin strip would also do...

The same thing that prevents them from just having glass down the entire length of the fuselage, rather than discrete windows: the ribs of the frame.
This picture from cpast's answer on UX.SE shows how the windows are placed relative to the frame:

On smaller aircraft, they probably could install small windows like the ones beside the passengers in the roof of the aisle, but it would add design complexity and also heat up the cabin (especially when sitting around on the ramp.) I'm assuming the cost/benefit ratio there was just deemed not worthwhile. Having them directly above the passengers wouldn't work because that's where the overhead storage bins are.
In the case of wide-body aircraft (like the one picture above,) note that the actual top of the fuselage is quite high. In addition to the overhead storage bins above the passengers, there is often other stuff between the ceiling of the passenger cabin and the actual top of the fuselage, such as crew rest quarters, pipes, cables, or, in the cases of the 747 and the A380, another entire passenger cabin.
An additional problem with this is that it would make it harder to control cabin lighting, unless the windows were electrically dimmable, like the new ones on the 787. Longer flights will usually want the passenger cabin to be dark-ish in order to accommodate the passengers who want to sleep. With the windows beside them, passengers can open or close their window depending on how much light they want. That wouldn't work for windows in the ceiling (since they would affect multiple rows of passengers.)
Further Reading
Window size limitations are also discussed in Why aren't airliner windows aligned with their seats? and the previously-linked question on UX.SE.

One other consideration is that the crown of the fuselage is under significant tension*, and glass does not have appreciable tensile strength.
*Think of the fuselage as a tube being held up midway along its length (i.e., by the wing). The weight of the fuselage and its contents will cause bending about the center wing box, putting the crown in tension, the keel in compression, and the sides under shear. The shear can be channeled around the window frames but normal loads would require far more structure for the same.

The structural arguments have already been discussed, but a further thing to consider is that there really is no point.
For the small windows on airliners (necessarily small for the structural reasons outlined) the only person who gets a decent view is the person in the window seat, who has a wide angle view by virtue of being right next to the window.
The person in the aisle seat (or the centre block of a widebody) is lucky if they catch a glimpse of land out of the window.
A window on the ceiling of an airliner, besides being to far from anyone to give anyone a decent view, would be be pointing straight up at the sky. All you would see would be blue sky, cloud (under certain weather conditions while below cruise height) and blazing hot sun.
I recently flew from London to Guanzhou, China, on a 787, on which the designers made the appalling high-tech decision of using dimming windows instead of traditional shutters. The windows don't dim all the way to black, and the sun shining through the window is a real distraction when you're trying to sleep. Who would control the shutters on roof windows in an airliner?


text:
How do I find a plane's history when booking a flight?
While booking tickets for a commercial airliner, is it possible to know the name of the aircraft (not the model, but the designated name) and then search the history of the plane, on how old is it, how many repairs has it gone through, has it ever had an accident etc.

At the point of booking, probably not - they can change the aircraft you're flying on the day. Just before you're bording would be more possible - you can often look out the window in the terminal and read the tail number.

And with the tail number, you may be able to find a part of its history on spooters website (e.g. the china's presidential aircraft)

Possible duplicate: http://aviation.stackexchange.com/questions/16855/how-far-ahead-does-an-airline-know-which-aircraft-you-will-be-flying-on

You can maybe guess:  Book a flight with Lufthansa from FRA to JFK, select a flight with a 747-8I (this is shown one the website for people like us, which prefer a special plane because the like it and so on), now you now that it can just be one of 19 differenct 747-8I. Now you can be at least sure, to fly on a pretty new plane:  https://docs.google.com/spreadsheets/d/1hD2Gkil30TiAIaV52BgTtfUrPyTdyi8qvCN59c_LHjU/pub#

Unless you are booking at the very last minute, no there is no way to find out which particular aircraft you'll be on until you're strapped in. The airline themselves plan in advance but as soon as one plane needs unexpected maintenance, or is excessively delayed, the schedule is thrown out the window. For this reason more than anything no airline to my knowledge publishes this schedule.
Once you are at the gate you might be able to see the aircraft registration. If you google that you can see if it has been in any serious incidents, when it was delivered and how old it is. You won't be able to find out how many repairs it has gone through, only the airline has that (hint: the answer could be hundreds of times if you're counting minor issues).